Abstract

During ontogeny, male rock shrimp Rhynchocinetes typus develop from the typus morphotype through several intermedius stages to the robustus morphotype, the last molt stage, which features powerful third maxillipeds and chelae. In a competition-free environment, all male stages mate in a similar manner. They guard females for 23 min to 3 h and perform several behaviors before and after they transfer spermatophores. In the present study, we observed the mating behavior of three different ontogenetic stages of male rock shrimp in a competitive environment in which the three different male stages competed directly for access to receptive females. In the first set of experiments, two males of different ontogenetic stages were placed together with a receptive female. The results indicated the existence of a linear dominance hierarchy, being robustus > intermedius > typus. In the second experiment, we examined the behavior of subordinate males while two dominant-stage males competed for a receptive female. During agonistic displays of the two dominant-stage males, the female escaped from the embrace of her (dominant) mating partner. Some subordinate males used this opportunity to pair with the female and mate rapidly without courting, contrasting to their courting in a competition-free situation. In a competitive situation, subordinate males that gained access to a female deposited several spermatophores as soon as possible (within 1 min). At the beginning of both experiments, subordinates that interacted first with the receptive female rapidly transferred spermatophores. Thus, in a competitive environment, subordinate males may use an alternative mating tactic that is characterized by speed. The social environment may impose divergent selective pressures favoring the evolution of conditional alternative mating tactics.